1. Field of the Invention
The present invention is related to the monitoring of radiation in a lithographic system and, in particular, to real time monitoring of radiation levels with minimal impact on radiation exposure within the lithographic system.
2. Background of the Invention
As the dimensions of semiconductor devices become smaller, so do the wavelengths of the radiation required in lithographic processes performed to deposit them. In order to efficiently print patterns of dimension 0.1 μm or less, lithography processes utilizing extreme ultra violet radiation (EUV) can be utilized. EUV utilizes light of wavelength about 10 to about 15 nm. The intensity and distribution of the EUV radiation applied to the resist should be controlled carefully in order to optimize the lithography process.
Ideally, the intensity and intensity distribution is monitored in the wafer plane by moving a detector or detector array into the EUV beam at the location of the wafer. However, this procedure significantly reduces the throughput of the lithography instrument. Further the beam properties would not be certain between measurements.
FIG. 1 illustrates a conventional lithographic instrument 100 that measures the exposing radiation intensity in the illumination region of the lithographic instrument prior to the radiation impinging on a reticle 107. As shown in FIG. 1, a radiation source 101 provides a beam of radiation. The radiation from source 101 is focused onto reticle 107 by illumination optics. Illumination optics includes focusing lenses 102 and 106. The pattern imprinted on reticle 107 is projected onto wafer 109 by projection optics 108.
A beam splitter 103 is inserted between lens 102 and 106 in order to reflect a small fraction of the illumination onto illumination detector 105. A focusing lens 104 is inserted between beam splitter 103 and illumination detector 105 configured to focus light onto illumination detector 105. Illumination detector 105 can measure either the total illumination intensity or the illumination intensity distribution or both.
However, because EUV radiation is strongly absorbed by all materials, placing beam splitter 103 in an EUV lithography system is not possible. Additionally, EUV illumination optics consists entirely of mirrors, for precisely the same reason. In some systems, the EUV illumination has been measured near the periphery of one of the mirrors utilized to direct the EUV radiation. However, this approach does not measure the intensity of the EUV radiation near the center of the mirrors and does not measure the intensity distribution of the radiation. In another approach, the EUV radiation was measured by a detector placed on the surface of one of the mirrors or through a hole drilled through the mirror. However, this approach again only provides a single measurement of the intensity and does not provide accurately either the total illumination intensity or the intensity distribution at the reticle.
Therefore, there is a need to provide a system to better measure the EUV intensity distribution of the EUV beam in an EUV lithography system.